Growth, properties, and reactivity of high-concentration oxygen phases on platinum(100)

Pt oxidation is of fundamental scientific interest and is critically important to many applications of heterogeneous catalysis since the active surface of a Pt catalyst can be in a variety of oxygen-covered states under the operating conditions of typical catalytically relevant processes. Given that such oxygen phases can significantly affect catalyst performance, there is substantial motivation for pursuing a detailed understanding of Pt oxidation. As such, we utilized gas-phase atomic oxygen beams and surface analysis techniques in ultrahigh vacuum to investigate the growth, properties, and reactivity of high-concentration oxygen phases on the Pt(100) surface.;We find that, for relatively low coverages, the types and relative populations of oxygen phases that develop are highly dependent on the surface temperature during adsorption. Indeed, a disordered oxygen state preferentially forms at 450 K, at the expense of a two-dimensional, surface oxide that forms at 575 K. Thus, the disordered state appears to be metastable relative to the two-dimensional oxide. For higher coverages, oxygen atoms apparently adsorb on top of the two-dimensional oxide and act as a precursor to forming bulk-like, three-dimensional oxide particles. We show that a model assuming these precursor oxygen atoms can either associatively desorb or react with the two-dimensional oxide to form a three-dimensional oxide particle quantitatively reproduces the measured kinetics governing the transition from two-dimensional to three-dimensional oxide growth.;By examining the reactivity of the oxygen-covered Pt(100) surfaces toward the oxidation of CO, we find that the reaction is facile on oxygen phases that form below coverages of about one monolayer and inefficient when three-dimensional Pt oxide particles partially cover the surface. Our work shows that the intrinsic reactivity toward CO is highest for the twodimensional oxide and lowest for the three-dimensional oxide. At higher temperatures, reaction also occurs on metallic regions of the surface covered with relatively low concentrations of chemisorbed oxygen atoms, while the morphology of the three-dimensional oxide strongly influences the kinetics. Our work supports the growing evidence that three-dimensional oxide on Pt surfaces is less reactive toward CO oxidation than the oxygen phases that form at intermediate coverages.